Retro-reflective material is mainly used for traffic signs, marking, sight guidance line and other traffic safety warning signs. It plays an important role in the protection of traffic safety, and retro-reflection characteristic is the most effective and direct method to evaluate this material. The retro-reflection measurement requires very small observation angle which is the angle between the retroreflector axis and the observation axis, therefore a long distance is often needed to precisely control the angle, moreover, the measurement is usually completed in a large dark room, which greatly limits its application in practical engineering measurement. For industrial production line and on-site rapid measurement, an annular band optical path is adopted to shorten the distance and meet the illumination/observation condition for retro-reflection measurement.
The U.S. Pat. No. 7,961,328 B2 disclosed a system and method for measuring the photometric retroreflectivity of materials. By the combination of an annular transparent portion and an annular mirrored portion, it defines two annular beams that realize the measurement in two observation angles. However, multiple support devices are needed to support the annular transparent portion and the annular mirrored portion, which can cause the incomplete of annular beam and affect measurement accuracy; furthermore, it increases the risk of instability of the mechanism system. The annular transparent portion also requires small size of beam splitter mirror which will produce larger energy loss, and will also introduce the measuring error and refraction process. Besides, the mall size mirror is difficult to be processed and fixed.